Description of various commands available for usage in the dockerfile

Dockerfile notes

References - 
1. https://www.howtoforge.com/tutorial/how-to-create-docker-images-with-dockerfile/
2. https://takacsmark.com/dockerfile-tutorial-by-example-dockerfile-best-practices-2018/
3. https://docs.docker.com/engine/reference/builder/
Notes - 
 - A script or receipe having collection of commands and instructions, which will be executed in sequence, to build a new docker image.
 - Below are some Dockerfile commands and their usage.

1. FROM
 - Base docker image to build new image on top. This is 1st command in the Dockerfile. E.g. dockerized ubuntu image.

2. MAINTAINER
 - Contains name, email of maintainer of the image. Name <email>

3. RUN
 - To execute shell command during image building process.
 - Runs in new layer and creates new image. Used mostly for installing packages.

4. ADD
 - Copy files or directories from host machine to the docker image. 
 - A URL for file can be passed instead of Host path, which will be downloaded to destination directory.
 - It also allows to exract local tar file into destination path.

5. ENV
 - Define shell environment variable. TODO: are they visible only in entrypoint shell or persisted across the Image?

6. CMD
 - Shell command to be executed when a new container is built from this image.
 - It is generally the last command in the dockerfile.
 - It can be overridden using command given in the docker-run.

7. ENTRYPOINT
 - It configures the command to run container as executable.
 - This along with corresponding parameters are not ignored when docker-run is given command line parameters.
 - Default is '/bin/sh -c'

8. WORKDIR
 - It sets the working directory for RUN, CMD, ENTRYPOINT, COPY, ADD instructions.

9. USER
 - Set the default user or UID for the container created with the image.

10. VOLUME
 - Add acess/linked directory between the container and the host machine.

11. EXPOSE
 - Tells docker that container listens on specified network ports at runtime and with which protocol.
 - Default protocol is TCP.
 - It doesn't actually publish the ports, but tells which ports are intended to be run.
 - Ports are actually published with '-p <>:<>' in the docker-run command.

12. COPY
 - Similar to ADD, except that remote file URLs are not allowed.
 - Best practices recommend to use COPY instead of ADD.